Upper limits of H2O in the exosphere of the Moon from SWI/JUICE during the Lunar-Earth Gravity Assist

The Submillimeter Wave Instrument (SWI) aboard the JUpiter Icy Moon Explorer (JUICE) satellite conducted a series of dedicated observations of the Moon and Earth atmosphere as part of an instrument characterization and calibration campaign during the Lunar-Earth Gravity Assist maneuver in mid-August 2024. The JUICE satellite achieved an altitude of approximately 700 km during its closest approach, while subsequent moon observations were conducted at distances of around 500,000 km. The lunar atmosphere was observed in both nadir and limb geometries in the two bands of SWI (600 and 1200 GHz). Although no direct signature of the water line was detected in the data, these observations allowed upper limits to be placed on the neutral water column density. We proceed to summarize the measurement, modeling approach, and its underlying assumptions, followed by a discussion of the implications for the moon's water cycle.